1. Field of the Invention
The present invention relates to a low alloy rotor steel which is suitable for use in rotating apparatus which operates at elevated temperatures. The steel possesses substantial freedom from temper embrittlement, exhibits a good creep resistance and possesses a composition which when controlled in accordance with the equations which are set forth hereinafter, will provide a predetermined fracture appearance transition temperature and will reliably predict the change in the fracture appearance transition temperature. The ability to select the fracture appearance transition temperature and hence the freedom from temper embrittlement is highly valuable where the steel is used for example in rotating apparatus for power generation.
2. Description of the Prior Art
There are two steels in commercial use today which find a wide application in rotating apparatus. One steel contains about 3.5% nickel, about 1.5% chromium, about 0.5% molybdenum, about 0.12% vanadium and the balance essentially iron with incidental impurities. The other steel has a nominal composition of about 1.25% chromium, about 0.5% molybdenum and about 0.1% vanadium. These steels when properly heat treated will possess an ultimate strength of about 140 kpsi, an elongation of about 15% and will demonstrate adequate creep strength at elevated temperatures.
These steels, however, suffer one significant drawback and that is that upon heat treatment in order to obtain the requisite strength and ductility, these prior art steels are usually heated to a temperature in the range of from about 1100.degree. F and up to the lower critical point of the steel. After such tempering heat treatment, the steels are air-cooled to room temperature.
Since it is impossible to quench large sections from the tempering temperature the toughness is adversely affected and this phenomena has been known in the trade as temper embrittlement. Temper embrittlement, of course, is that quality described as being a function of the change in the fracture appearance transition temperature which is readily manifested by the standard notched Charpy impact test. By heat treating samples of the composition in accordance with the manner in which the rotor steel is heat treated and by subjecting the heat treated steel samples to the Charpy impact test at various temperatures, usually within the range between about -200.degree. F to about +200.degree. F, it is seen that the fracture appearance transition temperature significantly increases and may rapidly approach temperatures higher than ambient temperature. At the same time the upper shelf energy may be decreased. The fracture appearance transition temperature can be readily ascertained by examining the fracture surface and visually determining where the fracture surface is characterized by about 50% brittle failure and 50% ductile failure during said Charpy impact testing. For a concise discussion of temper brittleness, see The Making, Shaping and Treating of Steel, ninth edition, published by United States Steel, page 1136 and The Metals Handbook, eighth edition, Volume II, entitled Heat Treating, Cleaning and Finishing, published by the American Society for Metals, page 56.
Heretofore, emphasis has been placed on the creep strength of the steel and it has been arbitrarily assumed that if the creep strength is high, this is the governing criteria for rotor steels. Such steels are exemplified by those set forth in U.S. Pat. No. 2,968,549 to Brady et al and U.S. Pat. No. 3,092,491 to Payson et al. While creep strength is a significant criteria for rotor steels operating at elevated temperature, nonetheless the toughness and more importantly the change in the fracture appearance transition temperature when the steels are maintained for extended periods of time at the embrittling temperature range and thereafter cooled to room temperature, is exceedingly important. This is especially so since the torsional stresses which the steel undergoes after cooling to room temperature or some lower temperature are significant when the machinery is once again placed in operation. If the toughness is low or the fracture appearance transition temperature has been changed so that the same is above room temperature, normal starting torques may be sufficient to fracture the rotor steel in attempting to start the rotating apparatus after cooling to room temperature. It is with this background in mind that the effect of the alloying element on the change in the fracture appearance transition temperature assumes critical relevance and control thereof is absolutely essential in comparison with the necessity for attaining the requiisite ultimate strength, ductility and creep strength commensurate with toughness.